Recently, the capacity of the optical CATV transmission system is remarkably increasing, and the number of television channels becomes as large as 100 to 150. Then, required characteristics to a semiconductor laser diode (LD, hereinafter), serving as a light source in the optical CATV system, gradually become higher and severer. For example, in an analog modulation circuit for an optical CATV of 150 chs, required characteristics of the LD are that, composite second order (CSO) distortion .ltoreq.-60 dB, composite triple beat (CTB) .ltoreq.-65 dB and output power (Po) .gtoreq.20 mW, and production yield of the LD satisfying the aforementioned requirements is very low at present.
In view point of the present situation, the present inventors have proposed a multi-LD array in which plural LDs are simultaneously modulated by the same electrical signal comprising a great many channels of television signals, and plural optical signals thus obtained are multiplexed by a multiplexer, and specification on distortions of individual LDs are effectively relaxed, because the light-power of each LD is decreased. According to the multi-LD array, the production yield of the LDs can be improved, because specifications on distortions of each of the LDs are relaxed.
However, several disadvantages have been pointed out on the conventional multi-LD array.
Since plural input electrodes for receiving an electrical modulating signal are provided on the respective LDs, the LDs are respectively connected to the same output terminal of a modulating signal generator by plural bonding wires. Then, the wiring inductances of the bonding wires and the parasitic capacitances of bonding pads arise in the input portion of the plural LDs, and the band width of a modulated optical signal is largely decreased.
Moreover, since the LDs in the conventional multi-LD array are connected in parallel, the input impedance of the multi-LD array is very low as compared with the output impedance of the modulating signal generator. Then, the greater part of a modulating signal power is consumed in a resistor inserted therebetween for impedance matching.